The invention relates to a method of producing optical fibers in which silica glass, with or without a dipant, is deposited on the inside of a silica glas tube. The silicon glass is deposited directly from the gas phase by the reaction of a nonisothermal plasma. After deposition, the tube is collapsed and drawn into a fiber. The optical fibers produced by such a method may consist of a core of doped silica glass and a cladding of undoped silica glass. Alternatively, the fiber may consist of a core of either undoped or doped silica glass, a first cladding layer of doped silica glass, and an outer cladding of undoped silica glass. The dopant, depending on its type, may increase or decrease the refractive index of silica. Dopants such as GeO.sub.2, Al.sub.2 O.sub.3, and TiO.sub.2 will increase the refractive index, while dopants such as B.sub.2 O.sub.3, or F will decrease the refractive index. In an optical fiber the refractive index of the core material is higher than the layer of glass surrounding the core. There may be a sudden increase (stepped index) or a parabolic increase (graded index) of the core refractive index.
U.S. Pat. No. 4,145,456, which is hereby incorporated by reference, describes a method of producing internally coated glass tubes from which optical fibers can be drawn. This method uses a nonisothermal plasma at a pressure of approximately 1.3 to 130 millibars. In order to obtain good, strain-free layers, it is necessary to heat the glass tube to produce a high temperature zone superimposed on the plasma. In this method vitreous layers are formed directly from the gas phase. Glass particles are not usually formed.
During the experiments which resulted in the invention, it was found that in the above-mentioned plasma method fine glass particles may possibly be formed when the reaction conditions are changed. However, such particles are absolutely imcompatible with the method described in U.S. Pat. No. 4,145,456 and must therefore be avoided in all circumstances. It was found that in this method such particles cannot be melted down to vitreous layers without melting the tube. In particular, attempts to increase the deposition rate may result in the formation of the undesired glass particles.
It was also found that at a fixed, predetermined deposition rate the inside diameter of the tube must be as small as possible, at the end of the coating procedure, to achieve a geometrically perfect collapse.